This application incorporates by reference of Taiwan application Serial No. 090128261, filed Nov. 14, 2001.
1. Field of the Invention
The invention relates in general to security printing, and more particularly to the composition of an invisible ink.
2. Description of the Related Art
Invisible ink systems have been used for centuries. As is known, a colorless liquid A is used to form images on a document which thus results in an invisible text. At the receiving end the images formed by the colorless liquid A are revealed or rendered visible by either changing the pH value or by applying a second or activating liquid B to the paper. The chemical reaction of liquid A and liquid B can be an oxidation-reduction reaction, complexation, precipitation or catalytic reaction.
The reaction of a liquid, containing (NH4)2S and a second liquid, including Ag(NO3) produces precipitation and is widely applied to the latent image technology. The application of the basic solution, including 0.5M of NaOH and the acidic solution, including 0.1M of HCl and indicator is another example. However, these two typical latent image technologies couldn""t be applied to ink-jet printing and the resultant documents have very limited period of storage.
U.S. Pat. No. 5,395,432 taught the use of certain ink-jet compositions for use in printing latent images on a substrate. In accordance with that patent, ink jet compositions comprising zinc chloride were taught for use in making latent images. U.S. Pat. No. 6,106,110 enables the printing of a secure image, which is invisible to the naked or unaided human eye in normal visible light or with the use of ultraviolet or infrared light illumination. The secure image should only be revealed on demand after a substrate is subjected to a subsequent process of image activation. However, the colorless thermal ink-jet ink contains zinc compounds and the main ingredient of the color developer is organic solvent with other organic compounds, which have high toxicity. Thus, the invisible inks disclosed by the two aforementioned inventions are not ideal for use in offices.
It is therefore an object of the invention to provide an invisible ink with long storage time and high safety.
According to a preferred embodiment of the invention, an invisible ink composition is disclosed. The invisible ink composition comprises a carrier as a solvent; and at least one microencapsulated leuco dye evenly dispersing in the carrier. The invisible ink becomes visible while the microencapsulated leuco dye is combined with a developer.
Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings.